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Órgano Oficial del Instituto Nacional de Psiquiatría Ramón de la Fuente Muñiz

2002, Number 1

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Salud Mental 2002; 25 (1)

Importancia de la sustracción de un factor de escala global de las potencias absolutas del electroencefalograma para la ubicación precisa de la actividad electroencefalográfica anormal

Ricardo GJ, Fernández-Bouzas A, Harmony T, Ballesteros A

Language: Spanish
References: 33
Page: 25-34
PDF size: 185.42 Kb.


ABSTRACT

Frequency analysis of the electroencephalogram (EEG) has proved to be a method of great importance for the evaluation of patients with brain lesions. Two different EEG recordings may show a similar appearance but they may differ greatly in overall amplitude, which is due to a difference in a multiplicative factor in the raw spectra (a Global Scale Factor: GSF). Its subtraction decreases the non-physiological variability and contributes to improve the diagnostical precision. Hence; knowing the diagnostic accuracy of this procedure in well defined structural lesions, will allow us to better study the cerebral dysfunctions present in mental diseases.
The objectives of this paper is to evaluate the degree of coincidence of the diagnostical precision in a group of patients with brain tumors, by using the measures of the Absolute Power Spectrum (AP) in the delta, theta and alpha bands, obtained with and without the subtraction of the GSF, as well as in the Relative Power (RP) in the same bands, using as the truth criteria the lesion showed by the Magnetic Resonance (MR). Twenty patients (13 women and 7 men; mean age = 57 years) with supratentorial brain tumors were studied. In every case, the lesion was corroborated through simple and contrasted MR. APs were obtained with and without the subtraction of the GSF, as well as the RP, in the above mentioned bands. In each one of the patients it was determined which one of the derivations could be affected by the lesion, considering the size of the tumor. In every one of the 19 derivations the proportion of true positives (TP) was analyzed in all the sample. The TP proportion was defined as the number of patients that had a lesion in the i-th derivation according to the MR, and which values of the considered parameters (AP or RP) were bigger than 1.96 (absolute value), divided by the number of subjects that had a lesion in the derivation, according to the RM. The mean values of the TP were compared between the two considered parameters; such values were obtained from the average of the TP through all derivations. The statistical procedure used was T-test for paired samples with a significance level of p ‹0.05. The averages of TP in the AP were higher with the subtraction of the GSF than without such subtraction, in every band and montage. The results of comparing RP with its homologous in the AP, with and without FEG subtraction, were not as homogeneous as when comparing the AP with vs. without subtraction. It was concluded that the subtraction of the GSF in the APs delta, theta and alpha must be performed, if a better localization of the dysfunctions in the EEG produced by the brain tumors is desired. It was also concluded that the RP must be used in the same bands as complement of the AP from which the GSF was subtracted.


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